FIG. 1 diagrammatically illustrates an AT&T-compatible digital carrier terminal or D4 channel bank 10, such as that manufactured by Adtran Corporation, Huntsville, Ala., through which digital services may be provided by a digital carrier telephone network to digital signalling equipment located at a customer's premises 20. The channel bank 10 typically contains a line interface unit (LIU) 12 which interfaces one end of a T1 (1.544 Mb/s) time division multiplex (TDM) digital communications link 14. A second end of T1 TDM link 14 may be connected to another office, such as one containing an operational support system (OSS) 16, located remotely with respect to the office in which D4 channel bank 10 is installed.
Also contained in the D4 channel bank 10 is an office channel unit data port (OCU-DP) 18, which is coupled to LIU 12 via an intra D4 bank PCM communications link 15. OCU-DP 18 is configured to support a plurality of (e.g. twenty-four) channel units, each of which is associated with a respective (64 Kb/s) time slot of the T1 TDM link 14. The OCU-DP 18 is operative to interface bipolar signals on respective portions 21T and 21R of a local four-wire DS0 metallic (copper) link 21, which is coupled to a digital data service termination (DDST) 22, terminating the metallic loop 21 with a data service unit/channel service unit (CSU/DSU) 23 located at the customer's premises 20.
The LIU 12 and OCU-DP 18 within the D4 channel bank 10 include respective transmit/receive buffers associated with the respective (bipolar/DS1/DS0) ports of a channel unit. These buffers are controlled by a resident supervisory control microcontroller for interfacing DS1-formatted data traffic from the T1 link side of the terminal, retiming the traffic as a bipolar data stream for transmission as a DS0 data stream from the D4 channel bank 10 to the customer's DSU/CSU site 20, and reconverting bipolar signals, supplied from the customer site 20 to OCU data port 18, into DS1 data frames for transmission over T1 link 14 to the remote site.
The component and bus architecture through which LIU 12 and OCU-DP 18 of D4 channel bank 10 communicate with one another by way of PCM communications link 15 is diagrammatically shown FIG. 2. As shown therein, a respective channel unit 19 within the OCU-DP 18 has a transmitter section 31, to which DS0 data from the four wire metallic loop 21 is supplied from the customer site termination equipment. The channel unit 19 further includes a receiver section 33, from which DS0 data is coupled to four wire metallic loop 21 for delivery to the customer site termination equipment. The channel unit's transmitter section 31 is coupled via a set of transmission links 41, 43 and 45 to a transmit unit 35. The transmission link 45, labelled as TDATA bus, is employed to transport serialized data bits from the transmitter section 31 of the channel unit 19 to the transmit unit 35, during a respective time slot assigned to that channel unit. Link 41 contains a set of transmit sequence control leads on which transmission control signals TX.sub.-- CNTL signals from transmit unit 35 are asserted for controlling the format of data transmissions from the channel unit 19 on the TDATA bus 45. Link 43 is a clock lead on which a transmit clock signal TD-CLK is asserted by transmitter section 31.
In response to the control and clock signals on links 41 and 43, the transmitter section 31 of the channel unit 19 decodes its respective channel select strobe and transmits data packets onto transmit data TDATA bus 45 in a respective one of a plurality (e.g. 24) multiplexed channel unit time slots of a multi-channel (e.g. 24 channel) unit digroup within the D4 bank. Pursuant to industry (AT&T-defined) communication standards, the channel select strobe occurs at an 8 KHz rate, so that with an eight bit byte being asserted for each strobe, a 64 Kb/s (DS0) channel is provided for a respective DS1 line. As data is serialized out over the TDATA bus 45, transmit unit 35 collects the 192 bits (comprised of eight bits from each of the (24) channel units), appends a framing bit, and outputs the resulting DS1-formatted (193 bit) PCM data stream onto TPCM link 51, and an associated transmit clock signal via TCLK link 53 to the LIU 12. The line interface unit 12 couples the formatted DS1 data onto the digital T1 carrier for transmission over link 14.
On the DS1 receive side, incoming T1 carrier signals from link 14 are received by line interface unit 12, and extended superframe format is converted into superframe formatted signals, as necessary. Payload or signalling bits are not altered. The DS1 data is asserted onto a receive RNPCM bus 61, which is coupled to receive unit 34 and to the receiver section 33 of each channel unit of the D4 channel bank. The DS1 clock within the T1 data is recovered by LIU 12 and applied as a recovered clock signal on RCLK link 63, which is also coupled to receive unit 34 and to the receiver section 33 of each channel unit 19.
The receive unit 34 synchronizes its timing with the DS1 framing pattern of the received signal and supplies channel unit control signals over RX.sub.-- CNTL link 65 to the receiver section 33 of each channel unit in the D4 bank. This allows each channel unit to decode its channel select strobe for the received data and to extract its corresponding byte of data from the associated time slot of RNPCM data bus 61.
It should be noted that in the D4 channel bank configuration of FIG. 2, all channel units in the D4 channel bank share the transmit TDATA bus and the receive RNPCM data bus 61, so that each channel unit has physical access to every DS0 time slot in a digroup. Actual time slot allotment is carried out by control and clock signals supplied by the transmit unit 35 for the transmit direction and by the receive unit 34 for the receive direction, as described above.
The '665 patent describes an upgrade or enhancement to the office channel unit data port (OCU-DP) and line interface unit (LIU) components of an existing D4 channel bank, that enables the RNPCM bus of the channel bank's internal communications link to be controllably tri-stated for bidirectional signalling capability. The bidirectional signalling format on the controllably tri-stated RNPCM signalling bus is defined to support both the transmission of incoming (received DS1) signalling traffic from the line interface unit to an office channel unit, and the exchange of performance-monitoring (PM) command and response messages related to the operation of a DS0 loop, in a manner that is transparent to digital services subscribers. Pursuant to this PM scheme, one or one or more performance aspects of the DS0 channel are monitored and stored, in each of the channel unit-to-DDST direction and the DDST-to-channel unit direction. DS0 channel quality-representative messages are then forwarded to the line interface unit over the bidirectional RNPCM bus and reported to a supervisory control unit external to the channel bank, in response to a command message from the line interface unit.
The '288 application describes a further modification of the data communication format on the RNPCM bus to include two additional non-normal data communication formats, referred to as RNPCM INIT data format and RNPCM SMART data format. The RNPCM INIT data format is employed during an initialization (INIT) mode of operation of an upgraded channel bank to convey initialization command messages from a `smart` (i.e. performance monitoring-capable) line interface unit to a smart channel unit. The RNPCM SMART format is used during a `Smart` mode of operation of an upgraded channel bank, to convey command messages from an upgraded, performance monitoring-capable, `smart` line interface unit to an upgraded, performance monitoring-capable `smart` channel unit, as well as to convey response messages from a smart channel unit to the smart line interface unit. It is also used to prevent the occurrence of false yellow alarms. An enhanced smart performance monitoring (PM) line interface unit portion of an upgraded D4 channel bank includes the same signalling, timing and control components as a conventional line interface unit, plus additional circuitry which effectively converts a conventional line interface unit into a smart performance-monitoring line interface unit (PM-LIU).
Because the enhanced D4 channel bank architectures described in each of the '665 patent and the '288 application provide substantially improved functionality with respect to a conventional channel bank architectures, especially the ability to conduct network maintenance operations (including connectivity configuration, performance and status monitoring) from a remote site, users of the improved equipment would now prefer, to the extent possible, to perform all equipment configuration, maintenance and operation status related tasks from a central supervisory facility, thereby minimizing the need for field personnel to travel to and perform local equipment maintenance and configuration assignments.
One principal concern of the system operator is the knowledge of what equipment is currently plugged into the backplane slots of the channel bank, and how both that equipment and its attendant network is configured. Although a D4 channel bank backplane has provision for multiple (e.g., 24) channel units, not every channel unit connector slot will necessarily have a channel unit installed at each position, and the knowledge of which channel bank slots are occupied is necessary for meeting customer service demands. In order to determine which channel unit connector slots of a channel bank are occupied it is currently necessary for maintenance personnel to gain physical access to the interior of the channel bank cabinet and visually inspect the channel bank's connector slot positions. It would be desirable to devise a technique that would permit such a determination to be conducted remotely, similar to the remote monitoring and control capabilities of the enhancements described in the '665 patent and the '288 application.